Basin-Scale Transport of Hydrothermal Iron, Manganese and Aluminum Across the Eastern South Pacific

Abstract:

The U.S. GEOTRACES Eastern Pacific Zonal Transect cruise (GEOTRACES GP06) examined the water-column distribution of trace elements between Peru and Tahiti, crossing the southern East Pacific Rise (EPR) midway along the cruise track. Shipboard measurements made along this ocean section reveal the mid-depth lateral transport of hydrothermal dissolved iron (dFe), manganese (dMn) and aluminum (dAl) over a distance of more than 4,000 km, from the EPR westward into the deep South Pacific basin. Post-cruise measurements of the conservative hydrothermal tracer helium-3 indicate the loss of at least 85% of the hydrothermal dFe over a distance of ~80 km west from the ridge axis, presumably as a result of oxidation, scavenging and precipitation. Further west of the ridge axis, dFe and excess helium-3 (³He_xs) are linearly correlated (r² = 0.99), showing a more than 4-fold conservative dilution of hydrothermal dFe over a distance of ~4000 km. This behavior may reflect the lateral transport of iron as relatively unreactive, colloidal oxyhydroxides; ongoing analyses of cruise samples by other groups will provide data to test this hypothesis. The loss of hydrothermal dMn relative to ³He_xs extends over a greater distance than for dFe, as far as ~250 km to the west of the ridge axis, beyond which dMn exhibits nearly conservative behavior. The hydrothermal dAl anomaly appears to extend over 3,000 km west of the EPR, and is not readily explained based on the known composition of ridge-axis vent fluids. The linear dFe versus ³He_xs relationship in the off-axis hydrothermal plume has a slope of 5.6 x 10⁶ mol/mol, which falls roughly between values estimated for the western South Pacific and the South Atlantic basins. If we assume that our dFe and ³He data are generally representative of mid-ocean ridge hydrothermal emissions, then the estimated global hydrothermal ³He efflux of 530 mol/y yields an effective hydrothermal dFe input of 2.9 Gmol/y to the ocean interior.